Pyrolyzed biosolid surface features promote a highly efficient oxygen reduction reaction

Abstract

Oxygen Reduction Reaction (ORR) catalysts were synthesized from biosolids by simple pyrolysis at 950 °C. Their surface features were modified by changing pyrolysis conditions (ramp and holding time). The obtained materials, after an extensive surface characterization, were tested as ORR catalysts in an alkaline medium. They exhibited a high catalytic activity with the number of electron transfer close to 4 (3.99) and a kinetic current density of 8 mA cm⁻². Their excellent performance was linked to the hierarchical porosity, high integrity of the carbon phase of an enhanced graphitization degree, and to the presence of catalytic centers. The samples prepared at a long pyrolysis time were very active in the ORR due to the developed micropores and the high graphitization degree of the carbon phase. Mild acid washing decreased the number of electron transfer by removing metal-based catalytic centers and by negatively affecting the integrity of the carbon phase covering the pore walls.